1. Field of the Invention
The present invention relates to detonating cord and, in particular, to low-energy detonating cord that is not subject to cut-off by a similar detonating cord that functions when in contact therewith.
Detonating cord is well-known in the art of initiating explosive signals and usually comprises a solid core of explosive material such as pentaerythritol tetranitrate (hereinafter “PETN”) enclosed within a single- or multilayer-jacket. Some detonating cords comprise a single layer jacket, e.g., a single layer of polyethylene extruded over the explosive core, but typically, the jacket comprises a textile tube in direct contact with the core of explosive materials and one or more additional jacket layers thereover to provide the desired tensile strength, resistance to deactivation by water, and other desired characteristics. Detonating cord has a variety of uses, including the transmission of a detonation signal along its length from one device to another. Frequently, on a blasting site, one length of detonating cord may come in contact with another, either inadvertently or by design. Such inadvertent contact may occur with one section of a length of detonating cord with another section of the same cord; at other times it occurs because one cord must cross over another in order to convey detonation signals to their respective destinations. Some detonating cords come into contact by design, e.g., by the tying of a knot between them, so that a signal on one cord can be transferred to another cord.
A common problem with the use of detonating cord in these ways is that, depending on the nature of the contact between them, a first functioning detonating cord (or section thereof) may “cut off” another length of detonating cord, i.e., it may sever the second length of cord without initiating a signal therein. Once severed, the detonating cord is incapable of functioning to convey a detonation signal to its intended target. Cut-off may occur when one length of detonating cord is disposed in close proximity, or in contact, with another in a manner that does not permit signal transfer from one cord (or section thereof) to another. As indicated above, tying a knot between two cords is typically sufficient to enable signal transfer from one cord to another, but often, disposing the cords in unknotted, side-by-side relation with each other, or at an acute angle relative to one another, will cause the output from one cord to sever, but not initiate, the other.
2. Related Art
U.S. Pat. No. 3,726,216, issued to Calder, Jr. et al on Apr. 10, 1973, and entitled “Detonation Device and Method For Making the Same”, discloses a detonating cord designed for signal transfer from one section of such cord to another by tying a knot between them. The occurrence of cut-off is reduced so that the cords need not be restricted to mutually perpendicular orientation. This is achieved through the use of core material having finely granulated particles. In the illustrated embodiment, the explosive core is surrounded by several jacket layers, including (from inward to outward) an inner layer of fibrous layer 54, a textile layer 56, another textile layer 58, a moisture-impervious barrier 60, a textile layer 62, another textile layer 64 and a water-repellant outer protective layer 66. The PETN core material has a fine granulation such that only 15 percent or less by weigh is retained on a 100 mesh sieve (column 5, lines 27-33). The core loading of such material is in the range of about 15 to 40 grains per foot (column 5, lines 37-40 and column 8, lines 43-48).
U.S. Pat. No. 3,311,056, issued to Noddin on Mar. 28, 1967, and entitled “Non-Rupturing Detonating Cords”, discloses a detonating cord comprising an explosive core encased within a polyurethane elastomer sheath. The polyurethane sheath may be in direct contact with the core or it may be separated therefrom by one or more layers of materials such as metal, plastic or fabric (see column 2, lines 46-52). The core loading may be from 1 to 400 grains per foot (column 3, lines 8-11). The polyurethane does not rupture when the cord functions, so the cord will not affect adjacent temperature-sensitive materials (column 3, lines 55-65).
U.S. Pat. No. 2,982,210, issued to Andrew et al on May 2, 1961, and entitled “Connecting Cord”, discloses a cord comprising a crystalline cap-sensitive high explosive core 1 enclosed within a metal sheath 2 at a loading of 0.1 to 2 grains per foot. The metal sheath 2 may or may not be covered by a non-metallic material 3 such as fabric or plastic (see column 2, lines 48-60). The cord will not initiate or damage another cord adjacent to it, or an adjacent dynamite cartridge; it can be tied in knots without interfering in the propagation of the detonation pulse and without a cut-off at the knot (see column 3, lines 35-40), and it can be used for bottom-hole priming (see column 5, lines 29-35).
U.S. Pat. No. 4,024,817, issued to Calder, Jr. et al on May 24, 1977, and entitled “Elongated Flexible Detonating Device”, discloses a detonating cord 20 having an outer energy-absorbing layer 30 releasably applied thereto (see column 9, lines 32-35). The energy-absorbing layer, which may comprise extruded plastic 72, is separated from the detonating cord therein by an intervening layer of fibrous material 70 such as cotton, rayon or other yarn (see column 9, lines 59-65). Therefore, the energy-absorbing layer 30 can be stripped from the detonating cord therein (see column 4, line 67 through column 5, line 3). The energy-absorbing layer and the layer of fibrous material 70 serve to dampen and reduce the transmitted energy available when the detonating cord is initiated, to prevent the detonation of explosive material which is in contact with the energy-absorbing layer 30 (see column 4, lines 24-28, lines 46-50 and column 5, lines 6-15). A short length of the energy-absorbing layer can be removed from the end of the detonating cord by circumferentially cutting the layer and slipping the severed portion of energy-absorbing layer off the end of the detonating cord (see column 10, lines 31-36).